I can take a crack at your CO² questions for you.

First I rounded up your grow area.

Grow room area: 36 cubic feet
Amount of CO2 required: 0.0432 cubic feet
On time: 2.59 minutes

At this flow rate: 1 CFH

If you are using a 20 pound CO2 bottle with a regulator, it will last 174.8 hours.

So at 1 CFH your regulator is on for about 3 min. This will bring the grow area well above the 1500-PPM and closer to 2000-ppm would be my guess. I rounded up the area remember?

The idea is to raise the level above 1500-ppm and hold it there for about 3-4 hours at a time. Then exhaust the room and exchange all the air and do it all over again until the lights go off.

Now holding it there is the trick because with out a PPM monitor you can't calculate leakage, plant uses ect. So you can only guess.

My best guess is run your timer for 3 min just before the top of each hour. Then every 4th hour you run an exhaust cycle for 8 mim at the end of the hour and give the room 2 min to pressure down before you start with the Co² all over again.

The whole time you are using Co² the lights are on and you have a fan moving the air in the cabinet around. The exhaust is off and the air intake is closed off also. You need to hold the level for as long as you can so try to keep it from leaking out.

Did that help or do you have other questions?